Typical variable pitch aircraft propulsor systems include a plurality of propeller blades mounted for pitch adjusting movement about each blade's longitudinal axis to a rotating hub driven by the aircraft engine. U.S. Pat. No. 4,523,891 shows such a system wherein the pitch adjusting linkage and mechanism is disposed within the hub which is located at the axis of rotation of the propulsor.
Modern ultra-high bypass ducted fan engines also use variable pitch fan blades for achieving efficient performance over a variety of operating conditions. Such systems may include a forward or booster compressor disposed upstream of the ducted, variable pitch fan which results in an annular flow of engine working fluid being conducted axially past the propulsor, radially inward of the variable pitch fan blades. Such propulsors typically have an annular hub, disposed radially outward of the working fluid stream and supported relative to the engine shaft by a plurality of radial struts which are secured to a radially inner bearing, or the like. The annular hub portion supports the individual propulsor blades.
Variable pitch positioning systems according to the prior art which are disposed adjacent the engine shaft require mechanical links or other blade pitch adjusting structure to pass radially through the annular working fluid stream, possibly diminishing engine efficiency due to blocking of the working fluid flow. Other considerations in using an axially central pitch change system include the physical size of the mechanism which may constrain the engine design as well as the lack of serviceability of the radially inwardly disposed structure.
What is needed is a pitch change system which is adapted to fit within an annular hub disposed outside of an annular stream of working fluid.